The sense of sweet taste is mediated by the sweet taste receptor, T1R2+T1R3, which are two closely related G protein-coupled receptors of T1Rs family. T1R2+T1R3 is a broadly tuned receptor responding to all classes of sweet compounds. T1Rs have a large extracellular domain, 7-transmembranes (7TMs), and a short C-terminal tail. The goal of this application is to elucidate structure-function relationships of human T1R3 using molecular biology and the sweet suppressing compound lactisole. I will take advantage of the fact that lactisole acts on human T1R3 7TMs, but not on mouse T1R3. Aim 1 is to identify the human-specific TM regions and residues that are critical for lactisole's inhibitory effect on human T1R3 using human/mouse T1R3 chimeras and site-directed mutagenesis. Aim 2 is to determine other residues comprising the lactisole binding pocket by referring to similar GPCR 7TMs structures including bovine rhodopsin, mGluRs, and the extracellular calcium sensing receptor. These findings will be used to refine a model of 7TMs of human T1R3. Knowledge of the structure/function relationship of T1R3 may assist in the development of novel, low caloric sweeteners to help fight against diet-related disorders such as obesity and diabetes.